I'm about 185lbs, 6'1 and have just put an order in for the 6' spitty. I'm an intermediate surfer too.
I was thinking that I have maybe gone a size too big, so for you I would defo go the 5'10.
I'm going off the premise that it's best not to go too big so you can bust the moves out when you're up and surfing, providing the wave isn't too big.
Also, make sure you have just enough paddle for the weak days.
I was tossing up the wait for the custom size 5'11 but the wait may be too loooooong.
Hope this helps...

510 or even the 508 has plenty of float and paddle for a 173lb. intermediate to advanced surfer. Your height and also what you're used to know may also factor. For instance if you're on the taller side (say 6'2" or taller) and/or are used to longer boards then 510 may work better. But if you're at or less than 6' in height and are okay with shorter boards -- maybe you dabble with the occasional fish type boards -- then I would definitely go shorter.

Elevons will definitely give you a little extra lift on those average conditions. Perfectly suited for flatter rockered boards
The Solus are a blast. The little center hatchet makes this setup to be very loose off the top and the base gives them nice drive http://www.youtube.com/watch?v=HUq4eKlYNSU